Whilst I haven't used the 70D myself and thus have no objective or practical ways of showing the differences, I can't imagine that its dynamic range, colour depth and IQ with the same lens are so much better than those of the 60D that it's worth twice the price if most of what you plan on shooting is long exposures and if you have little interest in video recording.

I am looking at upgrading to the the 70D but that is based it appearing to be better suited to action photography than the 60D not to mention its video mode.

When you said your 50D is a the end of its life, do you mean it is in need of repair but you don't wish to spend the money doing so or is it in working order but you feel there is something in particular that's lacking compared to its successors?

Its an interesting one, the 7D is at a good price right now and offers some great features. But cannot see the 60D or 7D giving you a huge improvement in night time photography and long exposures.Hopefully Popo or others will read this and add some more valuable insight.

Just to throw it out there you may really benefit from the 6D more though than a 70D. Have you considered the 6D?

For what you are into, the 6D`s low light ability and low light focus ability should be a huge improvement over the 50D. As you already have the 24-70 2.8 lens , going ff is possibly the best option.

I have got both the 7D and the 6D and the 7D is great for sports and wildlife stuff, but for what you described I would be seriously looking at the 6D.

Going to second the 6D suggestion. Yes, it's a bit of cash to buy into... but you don't have any crop-frame lenses and the 24-70 is perfect for a full-frame camera. You require low-light performance, and the 6D has it in spades... Definitely gets my vote.

going full frame would mean that i have to sell the wide angle lens, which i use quite often and seeing that there aren't any EF lenses which would complete my 10 - 200 range(without the 23mm), i do prefer not to sell it and go full frame, because it's also a "little" bit out of my budget right now.

I appreciate the budget bit, so a 60D might give you what you need short term. The 60D will make a good back up body , if the you do get the chance to get a 6D or eq. one day. Alternatively hang off for a bit and see if you stretch it to the 6D. For what you are looking you would appreciate the step up. Its an amazing step up in low light form the 7D, so a tad more still from the 50D.